Generally, in the case where acetic acid is prepared based on the methanol carbonylation reaction, the iodine compound is used as the reaction promoting agent in the form of methyl or hydrogen iodide (CH.sub.3 I or HI).
The iodide which is used as the reaction promoting agent is mostly recovered by distillation.
However, usually the iodine compound remains in the form of iodine ions I.sup.-, iodine molecules I.sub.2, or alkyl iodine (particularly methyl iodine) in small amounts of several or several scores ppm.
In the case where acetic acid in which such small amount of iodine compound remains is used as the raw material, if the catalyst is very sensitive to the small amount of the iodine compound, the concentration of the iodine compound used as the raw material has to be lowered to an extremely small amount (several scores ppb) so as for the iodine compound to be suitable to the process.
Thus it is required that the concentration of the iodine compound remaining in the acetic acid be lowered to an extremely low level.
As the conventional iodine or iodine compound (to be called iodides) removing methods, there is an oxidation method (U.S. Pat. No. 3,709,705), methods using chemicals as a scavenger (U.S. Pat. Nos. 4,246,195 and 4,664,753), a hydrogenation method (U.S. Pat. No. 4,792,420), a distillation method (U.S. Pat. No. 4,029,553), and a crystallizing method (Japanese Patent Laid-open No. Sho-50-126610).
In these methods, although there are some differences, the cost for the distillation or the cost for the chemical salts constitutes the major expenses, thereby making them uneconomical. Further, the regeneration of the chemical salts used is difficult.
Meanwhile, there is known a method in which the iodine compound is removed by using a solid adsorbent. This method is different from the above cited methods, and has an advantage that the expense for the distillation and the expense of the chemical substance can be saved.
The typical methods are: the method using ion exchange resin (U.S. Pat. No. 3,943,229), the method using zeolite with a metal supported therein (U.S. Pat. No. 4,088,737), the method using silica with a metal supported therein (U.S. Pat. No. 3,838,554), the method using a ceramic with triethylene diamine supported therein, and the method using an activated charcoal with SnI.sub.2 supported therein (J. Nucl. Science and Technol 9(4), 197, 1972). All the above methods remove the iodine compound by using the adsorbent.
However, in the case where the iodine compound is removed by using a solid adsorbent, not only the treatment of large amounts is impossible, but also the metal salts supported on the adsorbent or the chemical compounds are extracted, thereby contaminating the acetic acid.